Putnam's catchphrase 'Meanings ain't in the head', and the associated label 'semantic externalism', are not without ambiguity, as many authors have pointed out. My aim here is not to separate and discuss everything which 'semantic externalism' could reasonably mean, or even everything it has meant to philosophers, but rather to identify three different true and insightful things it can mean. I believe that having all three insights, and having them separate, can pay big dividends in the philosophies of language and modality, but I won't try to make a case for that here.
We can look at language and thought on three levels:
1. Local marks and noises, local neural and sensory events.
2. Sense; game and moves; conceptual system and configuration; intension; internal content.
3. Reference; extension; external content in abstraction from internal.
We can individuate thoughts and propositions according to 2 alone, 3 alone, or 2 and 3 together. For example, on 2 alone, 'I am here' is the same proposition when you an I utter it, or when I utter it in different locations - we are making the same sort of move in the same sort of language-game. On 3 alone, 'I am here' expresses the same proposition as 'John is at location X' when the former is uttered by John at location X - extension (reference) is the same. On 2 and 3 together, 'I am here' is distinct when uttered by different people and at different locations, due to difference in extension, and also distinct from 'John is at location X' when both are uttered by John at X, due to conceptual difference (difference in sense, intension, language-game).
To take another kind of example, on 2 alone, 'Hesperus is Phosphorus' is distinct from 'Hesperus is Phosphorus', but identical to the Twin Earthian thought or proposition 'Hesperus is Phosphorus' - call this thought or proposition 'Twin-"Hesperus is Phosphorus"'. On 3 alone, 'Hesperus is Phosphorus' expresses the same proposition as 'Hesperus is Hesperus', but not the same proposition as any thought about Twin Venus. On 2 and 3 together, 'Hesperus is Phosphorus' is distinct from 'Hesperus is Hesperus' (due to difference with respect to 2), as well as from Twin-'Hesperus is Phosphorus' (due to difference with respect to 3) and from Twin-'Hesperus is Hesperus' (due to difference with respect to both 2 and 3).
The three semantic externalisms which I want to identify and separate can all be seen as underdetermination theses. They are:
Wittgenstein externalism: 1 doesn't determine 2 (i.e. local happenings don't determine intension).
Intension-based Putnam externalism: 2 doesn't determine 3 (i.e. intension doesn't determine extension).
Happenings-based Putnam externalism: 1 doesn't determine 3 (i.e. local happenings don't determine extension).
Note that the first two externalisms don't imply the third - while the relation of determining is arguably transitive, the relation of not determining isn't. Note also that 'determine' here means 'always determine' or 'generally determine' - determination in some cases is not being ruled out. I should also say that I attach no great importance to the labels used here for the three externalisms.
Finally, note that the expression 'internal content' above is not supposed to express the problematic notion of narrow content. Narrow content is often thought of as a kind of minimal intension, sense or internal content which is determined by happenings inside an agent's brain or body. That problematic notion isn't being discussed here.
I am currently working on a book on modality, in which this separation plays a key role.
For Wittgenstein externalism: Wittgenstein's Philosophical Investigations. Also his Zettel, and other later work.
For the Putnam externalisms: Putnam's classic 1975 paper 'The meaning of 'meaning', published in Minnesota Studies in the Philosophy of Science 7:131-193, reprinted in The Twin Earth Chronicles: Twenty Years of Reflection on Hilary Putnam's ``the Meaning of `Meaning' '' edited by Andrew Pessin & Sanford Goldberg, 1996, published by M. E. Sharpe.